1. Field of the Invention
The present invention relates to an optical or optomagnetic type apparatus which optically records, reproduces or erases information on a disk-like optical information recording medium supported on a turn table by irradiating the medium with a plurality of beam spots.
2. Description of the Related Art
For optical information recording/reproduction apparatuses of this kind, an arrangement such as that shown in FIG. 1 for irradiating desired information tracks on an optical information recording medium with a plurality of beam spots has been adopted.
In this arrangement, a turn table 2 serves to support a disk-like optical information recording medium 1 and to drive and rotate the recording medium 1. An optical system for irradiating the disk-like optical information recording medium 1 on the turn table 2 with a plurality of beam spots is formed of a fixed optical system 3 fixedly provided on the apparatus, and a movable optical system including a mirror 5 mounted on a carriage (not shown) movable in a radial direction of the recording medium 1 by a linear motor (not shown) and an objective lens 6 driven in a radial direction of the recording medium 1 (beam spot tracking direction) by an actuator (not shown).
In the fixed optical system 3, two laser beams are emitted from a two-beam semiconductor laser array 301 and are led to an image rotating element 4 through a collimator lens 302 and a polarizing prism 303. After light images have been rotated on a reference optical path by the image rotating element 4, the beams are directed to the mirror 5. In the movable optical system, the mirror 5 reflects the laser beams in a direction perpendicular to the recording surface of the recording medium 1, and the objective lens 6 condenses the beams to form desired beam spots on the recording surface of the recording medium 1.
The light reflected from the recording medium 1 is returned from the movable optical system to the fixed optical system, and is led to the polarizing prism 303 via the image rotating element 4 and is led to light receiving elements 307a, 307b, and 307c through a condenser lens 305, a 1/2 wavelength plate 304, other polarizing Prism 303, a condenser lens 305 and a cylindrical lens 306. An image rotating prism is ordinarily used as the image rotating element 4.
The light receiving element 307a is used to obtain information for automatic power control of the 2-beam semiconductor laser array 301. One of the two beam spots on the recording medium 1 is used as a servo system for tracking and focusing of the beam spots on information tracks, and information for this servo system is obtained from the light receiving element 307b. Reproduction signals of read information are obtained from the light receiving element 307c.
It is possible to effect initial tracking adjustment of the beam spots on the recording surface of the recording medium 1 by rotating the iamge rotating element 4 in the directions of arrows c in FIG. 1. As a method of imaging the beam spots on information tracks, methods such as those shown in FIGS. 2 and 3 are known. If two beam spots are irradiated on the same information track as shown in FIG. 2, a direct-verification system is adopted in which information is written with a beam spot b and, immediately after this writing, the written information is checked by being read out with a beam spot a which follows the beam spot b. If two beams are irradiated on different tracks as shown in FIG. 3, a system for simultaneously reading/writing information with two beam spots is adopted.
To obtain sufficiently high recording or reproduction performance, it is necessary to adjust two beam spots to the desired information tracks at a precision on the submicron order. To do so, a mechanism for rotating the image rotating element 4 with high accuracy and high resolution is required. FIG. 4 shows a perspective view of a mechanism for rotating an image rotating prism provided as a conventional image rotating element 4. The image rotating prism 4 is bonded to a rotating base 401 which is rotatably supported on an attachment plate 403 fixed in the fixed optical system 3. Reduction gears 404 and a worm gear 406 are engaged with the attachment plate 403. The worm gear 406 engages with a threaded portion 402 formed in a rotating base 401. As the gear 405 is rotated with a wrench, the rotating base 401 is rotating at a speed reduced by the gears to rotate and move the beam spot by a small angle.
In the above-described conventional apparatus, however, initial fine tracking adjustment is effected by using only the mechanism for rotating the image rotating element and it is therefore necessary to increase the accuracy and adjustment sensitivity of the rotating mechanism. Therefore problems arise including a complicated construction of the apparatus, an increase in manufacturing cost and an increase in adjustment time. Also the number of reflecting surfaces in the optical path of the optical system is increased by one to cause a phase difference which causes a deterioration in the signal reproduction performance.